getDescriptor

By default looks for a nested class (conventionally named DescriptorImpl) implementing Descriptor and marked with Extension.

Gets the descriptor for this instance.

Descriptor is a singleton for every concrete Describable
implementation, so if a.getClass() == b.getClass() then by default
a.getDescriptor() == b.getDescriptor() as well.
(In rare cases a single implementation class may be used for instances with distinct descriptors.)